Undesired evaporative emissions may be identified using engine-off natural vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, during engine operation, heat may be rejected to the fuel tank resulting a rise in fuel tank pressure. When the engine is subsequently turned off, the temperature at the fuel tank starts to decrease with a corresponding drop in pressure. After a sufficient amount of time has elapsed over which the temperature continues to drop, the pressure may drop sufficiently to create a natural vacuum in the fuel tank. If the fuel tank is isolated at this time (that is, sealed from the engine and the atmosphere), the vacuum will hold steady. However, if there is a leak in the fuel system, the vacuum will dissipate to ambient pressure, a rate of the vacuum dissipation based on the size of the leak. In this way, fine leaks in the fuel system (such as those as small as 0.02″) can be identified and addressed.
However, EONV leak tests take a long time to run. As an example, detection of a 0.02″ orifice leak may take 45 mins to an hour, or longer. Due to time constraints, EONV tests are not run during vehicle production in an assembly plant. A time-consuming fine leak test may be forgone in favor of a faster gross leak test (such as those capable of detecting a 0.04″ orifice). In one example, the gross leak tests may be completed in less than ten seconds at an end of line station of the assembly plant.
Various attempts have been developed to enable a faster leak detection at an assembly plant, before the vehicle leaves the plant. One example approach is shown by Gates et al. in U.S. Pat. No. 4,791,805. Therein, the fuel tank is placed in a vacuum chamber while a test gas is injected into the tank. Leaks are detected by the presence of the test gas in the vacuum chamber, as detected by a sensor sensitive to the test gas. In still other approaches, a leak is detected based on a pressure change in the vacuum chamber.
However, the inventors herein have recognized potential issues with such systems. As one example, the approaches described above require additional components, such as additional vacuum chambers and vacuum pumps that add to system cost and complexity. As another example, even with the added components, the presence of fine leaks may not be detectable in the time frame available at a vehicle production line of an assembly plant (e.g., in the order of 10 seconds or less). Consequently, vehicles may leave the assembly plant with fine leaks left undetected. This can cause increased warranty issues at low vehicle mileage. For example, when an EONV test is run for a first time, after the vehicle has been received by a customer, a fine leak may be detected and a diagnostic code may be set.
The inventors herein have recognized that when a fuel tank is “green”, that is it has never been fueled with fuel hydrocarbons, such as at a vehicle assembly plant, a significant pressure may build up inside it. When fuel is dispensed into the virgin fuel tank full of air, the cooled sprayed fuel may instantly flash, generating high fuel tank pressure. This “noise” energy can be advantageously used to perform a fine leak detection while the vehicle is moving on an assembly line conveyor. An example method for leak detection may comprise, on only a first initial fuel tank filling event while a vehicle is on an assembly line, with no prior fuel filling event, filling the fuel tank to a threshold level and sealing the fuel tank; and indicating degradation of a fuel system based on a change in fuel tank pressure while the vehicle moves along the assembly line. In this way, fine leak detection may be performed at an assembly plant without adding cycle time to an end of line (EOL) station.
As one example, after a vehicle has been assembled at an assembly plant, each vehicle subsystem may be tested at distinct end of line stations. When the vehicle reaches a station where a first virgin fill event of a “green” fuel tank is performed, a fine leak test mode may be initiated. As such, the first fill event may be a first fill event from an initial assembly of the vehicle (or at least initial assembly of the fuel system) at an assembly plant in which the vehicle is built. At the time of the first fill event, the engine may be shutdown and may have been run before. That is, a first ever combustion event in the engine since the initial assembly of the vehicle may not have occurred yet. Therein, once a threshold fill level has been reached in the green fuel tank (e.g., when the fuel tank is 15-20% full), the fuel tank may be sealed by closing a vent valve, thereby trapping the pressure generated (due to fuel flashing) during the filling of the fuel tank. The vehicle then continues to move along the assembly line to one or more other EOL stations with the engine maintained off. As such, the vehicle may spend a significant amount of time (e.g., multiple minutes) on the moving line before it reaches the end of the conveyor where the engine is started for a very first time, providing ample time for monitoring the change in pressure at the fuel tank for a fine leak. For example, the generation and/or dissipation of a natural vacuum in the fuel tank may be monitored as the vehicle transits with the engine off at the assembly plant post a first fill event. Before the engine is cranked at a last EOL station and a first ever combustion event of the virgin engine is completed, the leak test may be terminated, and an indication of fuel system degradation may be provided if the pressure changed faster than expected over the duration of monitoring.
In this way, green fuel tank pressure may be advantageously leveraged to perform a fine leak detection at a vehicle assembly plant. The technical effect of using the high pressure generated in the fuel tank during a first ever fuel tank fill event is that the need for additional leak detection components, such as pressure pumps or vacuum chambers, is reduced. By monitoring the change in fuel tank pressure as the vehicle moves along the assembly line following the first fill event, the leak detection may be completed without adding cycle time to any end of line (EOL) station. In addition, errors in leak detection due to the effect of exhaust heat rejection on the liquid fuel in the fuel tank (during engine operation or immediately after an engine shut-down) are reduced. By using the noise generated in a green fuel tank for completing fine leak detection despite the time constraints at an assembly plant, the accuracy and reliability of leak tests performed at the assembly plant is improved. By enabling both fine and gross leak detection to be completed within the limited time at an assembly plant, premature warranty issues are reduced. Overall, vehicle evaporative emissions quality is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.